With the recent evolution of electronics to slimness, lightness and high performance and with the rapid increase in the application of built-up PCBs (printed circuit boards) to light, slim and small electronic products including wireless communication terminals, digital camcorders, mobile computers, etc., multilayer PCBs are very extensively used.
Multilayer PCBs enable interconnections to be achieved three dimensionally as well as in planes. Particularly, multilayer PCBs can improve the integration of functional devices, such as ICs (integrated circuit), LSICs (large scale integrated circuit), etc., allowing electronic products to have slimness, lightness, and high performance, to achieved structural integration of electric functions and to be produced in a significantly reduced assembly time period and at a low cost.
Almost all the built-up PCBs employed in such applications have via-holes through which interlayer connection is achieved. The recent tendency toward slimness and lightness is causing laser drills to be the new rising technology used for the formation of the via-holes.
Via-holes, corresponding to passageways for interlayer electric connection, were conventionally formed using mechanical drills. However, mechanical drilling results in too large a hole to form fine circuits, in addition to increasing the production cost. Nowadays, laser drilling is preferred.
Generally representative of new, smaller vias, called “microvias”, a blind via is drilled from the surface of a PCB and terminates within the substrate.
If the cross-sectional area that a via occupies decreases, the ability to utilize vias increases by a similar amount.
However, the reduction of via size to microvia size means that mechanical drilling is of no commercial use, implying the generalization of some alternative processes, such as removal by way of laser or plasma.
The removal of material is the result of an electrochemical reaction to laser pulse or plasma treatment, which is different from a cutting action or treatment. However, removal processes similar to laser pulse or plasma treatment eliminate materials around the central line as well.
Because it primarily results in round holes, such removal, whether using laser, plasma, or others, competes with mechanical drilling. The formation of the round holes is often called ‘drilling’ due to the removal of materials around the central line, and is thus termed “micro via drilling”.
With reference to FIGS. 1A to 1D, the formation of via-holes in a PCB using a conventional laser drilling process is schematically shown in a stepwise manner.
The conventional method for forming via holes in a PCB by laser drilling, as shown in FIGS. 1A to 1D, comprises a laser drilling process, a desmearing and copper plating process, and a circuit patterning process.
The formation of via holes in a PCB starts with the drilling of a copper clad laminate 101 of FIG. 1A to form via holes 102 which travel through two layers, as shown in FIG. 1B.
Next, as shown in FIG. 1C, a desmearing process is conducted, immediately followed by plating the inner walls of the holes with copper to form a copper-plated layer 103.
Subsequently, the copper-plated layer 103 is patterned to form a circuit.
As a rule, the via holes 102 require hole lands for electrical interconnection. However, the hole lands of via holes act as hindrances to the increase of the degree of integration of circuit patterns. Referring to FIG. 2A, which shows conventional via-holes in a plan view, hole lands 104a˜104c prevent circuit lines 105a˜105c from being arranged near each other. As shown in FIG. 2A, although the via holes are disposed in a zigzag manner in order to reduce the distance between circuit lines 105a˜105c, the hole lands 104a˜104c still interfere with the approach of the circuit line 105a˜105c to each other.
FIG. 2B is a perspective view showing a via hole according to a conventional technology. The via hole, as shown in FIG. 2B, comprises a hole inner wall 107 associated with an upper hole land 104u at its upper portion and with a down hole land 104d at its lower portion. The upper hole land 104u is extended through a circuit line 105 to a wire bonding pad 106 while the down hole land 104d is associated with a solder ball pad 108. As can be seen, the hole lands 104u and 104d still occupy large areas.